Polk Audio db691 in motorcycle bags

Hi all! I've been lurking here for awhile and finally decided to post. I would say I'm a pretty good installer. I've been doing this off and on for years and have a pretty good understanding of what it takes to make something sound good in cars and trucks.

Recently a friend commissioned me to design and install a system on an '06 Kawasaki Vulcan Nomad motorcycle. It's kind of the Kaw version of a Harley Road King. It has no front fairing so no head unit and it does have hard bags. He wants to run all audio off and Ipod so I purchased the following;
Polk db521 5.25s, Polk db691 6x9s, Rockford PBR300x4 amp (75x4 rms), JL Audio CL-RLC remote level control/line driver and of course all the wires and cables needed including an Ipod digital to RCA cable.

I made custom pods to house the db521s and mounted to the crash bar up front. The db691s are in the bags (1 per bag) and the amp is in the right bag as well. The line driver is hidden behind one of the side covers. Everything is very neat and actually sounds amazing but there is one issue,..the 6x9s.

Being 6x9s are designed for free air setups in a deck they like a lot of air space. The output with the bags closed is minimal, muffled even. But crack the bags open 3/4" or so and it becomes outstanding! Before going further I should mention that the Kaw bags are side loading unlike a Harley which are top loading. The doors hinge at the bottom and the door lays open.

So why do I have so much sound on the backside of the cone? Do I need to come up with a way to port the bags? If I do port the bags am I construction a port to let air in or is the purpose of the port to vent the sound that is being created behind the cone?